Bug 1405808 - [mozlog] Default manifest groups to empty dict in errorsummary formatter, r=gbrown

This fixes an exception when a test_status/test_end is logged before a
suite_start. This case should be an error anyway, but might as well fix this to
keep the logs looking clean.

MozReview-Commit-ID: 2TlWlSmczwH

--HG--
extra : rebase_source : c33aed0870d7b7fa51d855383d6336331d4f22fc

testing/mozbase/mozlog/mozlog/formatters/errorsummary.py

@@ -10,7 +10,7 @@ from base import BaseFormatter
 class ErrorSummaryFormatter(BaseFormatter):
 
     def __init__(self):
-        self.groups = None
+        self.groups = {}
         self.line_count = 0
 
     def __call__(self, data):